Traditional abrasive materials used for precise polishing of glass optical elements, glass substrates, and semiconductor devices in their production processes are oxides of rare earth elements, mainly composed of cerium oxide and additionally containing lanthanum oxide, neodymium oxide, or praseodymium oxide. Although other abrasive materials, such as diamond, iron oxide, aluminum oxide, zirconium oxide, and colloidal silica, are also used, abrasive materials containing cerium oxide have been widely used due to their advantages such as a high polishing rate and low surface roughness of polished workpieces (surface smoothness thereof).
Most of commercially available cerium oxide particles as abrasive materials are typically prepared by a pulverization process. Such abrasive particles prepared by a pulverization process have sharply edged surfaces. These sharp edges of the particles, although increasing the polishing rate, readily scratch the surfaces of workpieces.
Glass optical elements, glass substrates, and semiconductor devices requiring high smoothness in order of angstrom (Å) are typically polished in two steps, primary polishing with cerium oxide particles having a high polishing rate, and then secondary polishing with colloidal silica of several tens of nanometers in size, to improve the smoothness of the surfaces (surface roughness) of workpieces.
Unfortunately, such a multi-stage polishing process reduces productivity. Due to an increasing requirement on the surface smoothness of workpieces, spherical abrasive particles should be developed which maintain a high polishing rate, and at the same time, barely cause scratch impairing the smoothness of the workpieces.
High-purity cerium oxide-based abrasive materials enabling precise polishing in the production process of optical glass are prepared by methods that involves adding a salt, such as carbonic acid, oxalic acid, or acetic acid, to an aqueous cerium solution containing purified cerous nitrate, cerous chloride, or cerous sulfate to precipitate a product, such as cerous carbonate, cerous oxalate, or cerous acetate, extracting this precipitate through filtration, and drying and firing the precipitate to prepare cerium oxide.
For example, NPL (non-patent literature) 1 describes a method that involves adding an aqueous urea solution as a precipitant to an aqueous solution of a rare earth element, such as an aqueous solution of cerium nitrate or yttrium nitrate, and heating the solution with stirring to prepare a precursor of abrasive particles having a narrow particle size distribution.
The present inventors fired the precursor of cerium oxide particles prepared by the method described in NPL 1 to form cerium oxide abrasive particles, and verified the polishing effect thereof. Unfortunately, the resulting abrasive particles had an insufficient polishing rate, which was not suitable for practical use. The present inventors clarified that the polishing rate was reduced due to a rare earth element other than the cerium element (such as yttrium) mixed to adjust the shape of the particle and the particle size distribution; this additional rare earth element relatively reduced the cerium concentration on the surfaces of the particles, and in turn, reduced the polishing rate.
PTL (Patent Literature) 1 discloses a polishing method with a composite particle composed of an organic particle and an inorganic particle applied onto the surface of the organic particle. Unfortunately, the composite particle prepared by the method disclosed in PTL 1 has a thin coating layer of the inorganic particle disposed over the surface of the organic particle, and oxygen barely diffuses inside such an inorganic particle layer, so that a large amount of trivalent cerium cannot be present on the surface of the particle. Additionally, the composite particle is significantly large, and cannot attain high surface smoothness of workpieces.
PTL 1 discloses a method of preparing a metal oxide particle prepared by heating a mixture of a metal salt, a high-molecular compound, and a high boiling point organic solvent to generate a metal oxide, and firing the metal oxide.
It is believed that cerium oxide particles typically used in polishing are composed of a large amount of trivalent cerium present on the surface of the cerium oxide particles and quadrivalent cerium stably present inside the particles, and the molecular bonds of the workpiece are broken by trivalent cerium present on the surfaces of the cerium oxide particles to progress polishing. This difference in valence of cerium between the surface and the inside of the particles, however, is small in agglomerates of nanoparticles, and trivalent cerium is barely present on the surfaces of the agglomerates. Accordingly, an increase in polishing rate is not expected.
It is believed as above that trivalent cerium contributes to polishing characteristics. The quadrivalent cerium, however, is more stable, and the unstable trivalent cerium cannot remain in the abrasive particles.
To solve this problem, PTLs 2 and 3 disclose methods of increasing the proportion of trivalent cerium in the structure of cerium oxide by adding impurities to a cerium compound to form a perovskite oxide.
Unfortunately, the methods disclosed in PTLs 2 and 3 cannot stably produce perovskite compounds in a large quantity, and a high proportion of impurities used in the methods precludes location of a large amount of trivalent cerium near the surfaces of the perovskite compounds.
PTL 4 discloses a method of preparing a metal oxide particle prepared by heating a mixture of a metal salt, a high-molecular compound, and a high boiling point organic solvent to generate a metal oxide, and firing the metal oxide.
Unfortunately, in the method disclosed in PTL 4, crystallites are aggregated into a particle. The particle is not spherical, and has an irregular surface to readily cause scratch on workpieces. Agglomerates of the aggregated particles composed of crystallites readily crash during polishing.
Furthermore, use of an organic solvent as a solvent requires a reaction at high temperature, reducing productivity. The high-molecular compound remaining on the surfaces of the particles aggregates during firing, leading to technical difficulties in control of the particle size.